Armed plainclothes "air marshals" will fly on UK passenger planes in an effort to deter and counter the threat of terrorism, the government announced today.

The capability "to place covert, specially-trained armed police officers aboard UK civil aircraft now exists", said the transport secretary, Alistair Darling. He added that bringing them in would be "a sensible and measured addition to the range of security measures we have available for addressing the threat to UK interests and to UK aviation".

The Department for Transport, which is responsible for overseeing the national aviation programme, was not giving out any details of what flights the marshals would operate on or what arms they would carry. The department did say that the trained officers would fly on both international and domestic UK flights.

"We are not discussing details because we don't want to give terrorists a chance to guess our intentions," said a department spokesman. He would not say whether any marshals had already flown on flights but added: "If you're a passenger on a flight from now on, you can assume that an armed officer could be on board."

The airline industry has been cool in the past when the idea has been mooted, with critics suggesting there was a risk of "sky marshals" being overpowered, or of gunfire perforating an aircraft's fuselage and causing decompression.

A new highly secretive scheme in Australia involves plainclothes officers with low-velocity firearms, chosen to reduce chances of puncturing a plane's interior walls.

The home office said earlier this month that airlines would be asked to foot some of the cost of air marshals.

Mr Darling said the move to place officers on planes followed a government decision earlier this year to reinforce in-flight security as part of the continuing review of aviation security.

He stressed that although the threat to UK aviation remained "a real one", the new measure had not been developed, nor had been announced, "in response to any new or specific intelligence".

Mr Darling went on: "The government will continue to work with the UK airline industry on sustainable measures for responding to the terrorist threat."

Israeli airline El Al has had air marshals for years, and the US has increased the number of gun guards after September 11. In the Australian scheme, 70 undercover guards fly on Australia's domestic routes while a further 40 will be on planes out of the country within weeks. It costs around £50m a year.

The normal scenario is for air marshals to sit near the front of the plane to protect the cockpit.

Mr Darling said that the measure enhanced moves already taken to increase security both on the ground at airports and in flight since the September 11 attacks in the USA. He said: "We have stepped up the search regimes for staff, passengers and their hand and hold baggage, vehicles, cargo and catering - with a particular emphasis on flights going to key destinations such as the USA."

He said the UK had added to the range of articles not allowed on aircraft and more money had been allocated for airport policing.

He went on: "On in-flight security, we are moving faster than the international community at large to ensure UK aircraft are fitted with intrusion-resistant flight-deck doors, and last month we acted to ensure that flight-deck doors on foreign aircraft are kept locked, as they have been on UK aircraft since very soon after the US attacks.

"We have also placed strict limits on those able to be on the flight deck of UK aircraft."

Stelios Haji-Ioannou, the former chairman of easyJet, said earlier this month he had qualms about air marshals. He said: "I am a bit worried about guns on planes ... it's the last place you want guns. What if the marshals are over-powered, or what if the guns fall into the wrong hands?"